It is about the zink content in
oil. There is only about 4 brands of
oil out there that fit the bill. Delo 400 15-40 is the best for the money. If you are runing solids and triple valve springs, after break in check the bottom of the lifter to the cam, if it shows any where (that is where the rubber meets the road) you need to change them and up your zink.
Yes
synthetic may give you more viscosity at higher heat( 300+) but if you are runing that hot you have other problems.
AF Morrow